3 August 2012

British Museum

With all these loose coins on my hands it seemed time to try cleaning some of them. As soon as we knew we were onto a hoard I contacted Julia Tubman of the British Museum Conservation Department.

With all these loose coins on my hands it seemed time to try cleaning some of them. As soon as we knew we were onto a hoard I contacted Julia Tubman of the British Museum Conservation Department. She and her colleagues are working on the Bath roman coin hoard and she very kindly had given me a lot of information about how they treated their coins. As soon as the hoard was up I went over to see them and it was great to see the techniques being used and to get loads of tips which I otherwise would have had to work out myself. Our hoard coins are made from a mixture of about 40% silver and 60% copper. They would have originally looked shiny and indistinguishable from pure silver but two thousand years buried in the soil has made the copper corrode, giving them a detail-obscuring lumpy green coating. The British Museum treatment, whch I adopted, uses a dilute formic acid bath to strip away the green copper corrosion and reveal the original silver surface. I tried this and it worked very well, allowing us to see all the original surface detail that had been hidden. As the BM had intimated however it has to be a very "hands on" process, as coins react at different rates and if the reaction goes too far copper can come out the acid solution and start to plate the coins. Therefore I found I could only do a small number at a time. After stripping, the coins are thoroughly washed again and again to remove any traces of the acid which might make them corrode even after they had been dried.

This is the one point in this blog where I will do the usual museum warning.

PLEASE DO NOT TRY THIS YOURSELVES. I KNOW FROM TRYING IT HOW EASY IT IS TO GO TOO FAR AND DAMAGE COINS. IF YOU HAVE ANY HISTORIC METALWORK TAKE IT TO A MUSEUM AND ASK THEM. DO NOT TRY DIY CONSERVATION, YOU'LL RUIN IT.

I showed images of the cleaned coins to our coin expert colleague Dr Philip De Jersey (he must get some stick living in Guernsey with that name!) and he said that they were all Coriosolite coins. This was the tribe who controlled the area of the french coast closest to Jersey at the time of the roman invasion.